Prevent InstantJobRegistry from altering the content of JobRegistry

[nulltoken]

Pull request description

InstantJobRegistry role is to trigger jobs not to mess with the registry.
It's not its responsibility and may potentially generate issue in the context of #108 and job definition unicity.

This PR:

• removes the update of the registry storage when a previously unregistered job is triggered and add a unit test to protect us from reappearing later.
• includes as a free bonus track the removal of an unused JobRegistry method.

PR meta checklist

• Pull request is targeted at main branch for code
• Pull request is linked to all related issues, if any.

Code PR specific checklist

• My code follows the code style of this project and AspNetCore coding guidelines.
• My change requires a change to the documentation.
  • I have updated the documentation accordingly.
• I have updated the appropriate sub section in the CHANGELOG.md.
• I have added, updated or removed tests to according to my changes.
  • All tests passed.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Files with missing lines	Coverage Δ
src/NCronJob/Registry/IInstantJobRegistry.cs	90.56% <100.00%> (-0.18%)	⬇️
src/NCronJob/Registry/JobRegistry.cs	91.20% <ø> (+0.88%)	⬆️

[nulltoken]

@linkdotnet Changelog has been updated

[linkdotnet]

Awesome - feel free to merge.